Dispersion alloy products of tungsten are well known wherein a tungsten grain structure in the final product such as a wire, a coil, or other type incandescent lamp filament is obtained having a non-sag grain structure attributable to incorporation of various additives. For example, U.S. Pat. No. 1,410,499 discloses various graingrowth promoting additives such as aluminum, silicon, and potassium compounds which can be uniformly distributed in the initial tungsten powder to achieve the desired objective. It is also known to vapor deposit a volatile tungsten compound together with the dopant materials on a suitable tungsten substrate as well as to vapor deposit the volatile tungsten compound by itself. An early recognition of the type tungsten crystal structure obtained by such vapor deposition appears in U.S. Pat. No. 1,149,701 which describes such tungsten deposit obtained by vacuum flash technique as long crystals lying radially with respect to the longitudinal axis of a tungsten incandescent lamp filament. Subsequent drawing of said composite wire product in the conventional manner to effect size reduction is said to produce a final lamp filament exhibiting improved ductility attributed to the fibrous nature and purity of the tungsten deposit.
Despite prior art recognition for some time that vapor deposited tungsten coatings can be used to improve incandescent lamp filaments, however, there has not been any major use made of such technology as a practical matter. The benefits obtained with the prior art coatings thereby apparently have not yet justified any additional effort to modify conventional incandescent lamp filaments in this manner. Consequently, it would increase the general utilization of vapor deposited tungsten coatings if additional significant performance benefits could be imparted thereby to incandescent lamp filaments, especially tungsten filaments.